Microstructure And Phase Transformation Mechanism Of The Surface Layer Of Titanium Alloy In High Speed Cutting

Ti-6Al-4V alloy has been widely used in aerospace and other fields due to its excellent properties such as high strength,low density,high temperature resistance,corrosion resistance and good low temperature performance.However,due to its inherent physical and mechanical properties such as poor thermal conductivity,low deformation coefficient and elastic modulus,it has become a difficult-to-process material.In this paper,the milling and turning experiments of titanium alloy Ti-6Al-4V were carried out respectively to study the effect of cutting speed on cutting force.The results show that with the cutting speed increasing in higher cutting speed,the cutting force tends to decrease.In addition,the change of surface hardness and residual stress with cutting parameters is also studied.The results show that the microhardness of the substrate is lower than that of the machining surface of titanium alloy,due to the work hardening of the surface layer.The depth of the work hardened layer is less than 120?m,but the microhardness is the highest at 20?m,and the heat softening occurs on the machining surface.What is more,the increase in cutting speed and feed leads to a gradual decrease in the microhardness of the machined surface.Under the cutting action,residual compressive stress appears on the machined surface.With the cutting speed increasing,the value of residual compressive stress tend to increase gradually.The microstructure of surface layer in high speed machining of titanium alloy was measured and characterized.The effects of cutting parameters on plastic deformation,grain size,grain boundary,orientation difference and phase distribution of surface layer were studied.SEM and EBSD experiments were introduced to study the microstructure evolution of the machined surface layer of titanium alloy.According to the results,the surface grain of the Ti-6Al-4V cutting layer is seriously twisted and stretched under the action of the cutting tool and produces refinement.When the cutting speed or feed rate increases,the value and thickness of plastic deformation layer increases.Moreover,under the action of cutting heat,the grains of the processed surface layer recover and recrystallize,making the grain size of the subsurface layer more broken than that of the surface layer.The increase of cutting speed and feed can promote further grain refinement,but the effect of cutting speed on grain refinement is greater than that of feed.Cutting parameters have an effect on grain orientation difference.With the increase of cutting speed,the frequency of grain boundary orientation difference of small Angle increases,and that of grain boundary orientation difference of large Angle decreases.With the feed increasing,the frequency of orientation difference has no obvious change.Under the cutting action,the deformation of ? phase is more severe than that of a phase transformation.XRD pattern and phase ratio analysis of cutting surface layer material were carried out to reveal the mechanism of phase transformation in titanium alloy cutting.The effect of cutting condition on the surface phase of titanium alloy was researched by XRD method.The results indicate that under the action of thermal-mechanical coupling,the surface of titanium alloy has a phase change and the phase ratio has a change.Compared with the matrix,the content of a phase decreases while the content of ? phase increases.And the diffraction peak in the XRD pattern is shifted.